Keyboards of the type used with computers or electronic typewriters comprise a pluarlity of key switch assemblies, one assembly being provided for each key position on the keyboard. A normally open switch is associated with each key switch assembly so that when a particular key is depressed, the associated switch is closed. The switch may be a membrane switch and the pulses or signals produced by depression of the keys are interpreted by the circuitry to produce the desired result.
Most key switch assemblies and the associated switches are designed such that when a particular key is depressed, the associated switch will be closed during downward movement of the key and will remain closed for a substantial time interval which extends from portion of the downward stroke of the key through a portion of the return stroke. When the operator depresses keys in succession at a rapid rate, it frequently happens that two or more switches will be closed at the same time; in other words, the closing intervals will overlap as between two keys which are depressed one after the other. When this happens, a false signal may be sent to the equipment associated with, or operated by, the keyboard and serious problems can result. For example, erroneous information might be generated and stored in a computer.
This problem has long been recognized and it is usually solved by building into the electronic equipment associated with the keyboard the ability to detect the operator's intention and transmit signals or pulses in response to the intention rather than in response to the signals which might be produced when two keys are depressed in rapid succession so that the associated switches are closed during an overlap time interval. This feature is usually referred to as N key rollover and is described, for example, in U.S. Pat. No. 3,745,536.
The present invention is directed to the achievement of a key switch assembly which has the N key rollover feature purely by virtue of its mechanical features so that it is unnecessary in the use of the keyboard to provide N key rollover features in the integrated circuits with which the keyboard is used.
A key switch assembly in accordance with the invention comprises a panel-like support, a pair of spaced-apart switch contacts on the support at a switch site and a frame on the support proximate to the switch site. The frame comprises a column extending normally of the support having a fixed end and a free end. A plunger is provided on the column which is normally maintained in an extended position relative to the support by a biasing spring and which is movably towards the support to a depressed position. A switch closing lever is pivotally mounted on the frame and is movable in response to movement of the plunger. The closing lever is effective electrically to connect the switch contacts to each other when it is moved. The key switch assembly is characterized in that the switch closing lever is a bell crank having a pivotal axis, a switch closing arm, and a cam follower arm. The pivotal axis is adjacent to the switch site and extends parallel to the plane of the panel-like support. The switch closing arm extends from the pivotal axis towards the switch site and has a free end which is proximate to, but normally spaced from, the switch contacts. The camming arm extends from the pivotal axis beside the plunger and has a free end which is spaced from the pivotal axis. The camming arm has a cam follower portion thereon. The plunger has a stiffly flexible camming rod extending from the upper end thereof towards the lower end. The camming rod has a camming portion which is angularly spaced from the cam follower portion of the camming arm, relative to the axis of the plunger, and which is moved past the cam follower portion during movement of the plunger to the depressed position. A cam deflector is provided on the frame proximate to the cam follower portion of the camming arm. The cam deflector has a deflecting surface which is engaged by the camming portion of the camming rod when the plunger is moved to the depressed position. The deflecting surface causes flexing of the camming rod laterally towards the cam follower portion of the camming arm with resulting engagement of the cam follower portion by the camming portion of the camming rod and resulting arcuate movement of the bell crank whereby the free end of the switch closing arm is moved towards the switch contacts and causes closing of the switch. The deflecting surface is of limited extent so that the camming rod returns to its normal position when the camming portion moves past the deflecting portion with resulting movement of the free end of the switch closing arm away from the switch contacts and opening of the switch. The switch contacts are thus electrically connected to each other only during a brief interval during movement of the plunger from the extended position to the depressed position.
In accordance with further embodiments, the switch contacts are opposed contacts on the substrate and the flexible membrane of a membrane switch. The switch closing lever is movably against the flexible membrane during movement of the plunger to the depressed position and is effective to flex the membrane towards the substrate and thereby engage the switch contacts with each other. In accordance with further embodiments, the frame has a base, the fixed end of the column is integral with the base, and the pivotal axis of the bell crank is in the base proximate to the fixed end of the column. The column has an axially extending opening therein extending from the fixed end to the free end and the plunger is slidably contained in the axially extending opening.